The present invention relates to adhesive tapes and to methods of making and using the same. More particularly, the present invention contemplates adhesive tapes useful to detachably join elements together, especially at least one flexible element. The tapes are especially useful in printing applications for detachably mounting printing plates in printing presses during printing operations.
In flexographic printing, flexible printing plates carrying the image to be printed are mounted to printing press cylinders for resilient printing contact with substrates to be printed. The printing plates are formed of polymeric materials such as rubber or photopolymer, and they are removably mounted to the steel print cylinders by double sided tape sometimes referred to as xe2x80x9cstickyback.xe2x80x9d In printing applications for both labelling and packaging, the printing plates are reused and several different plates may be used in daily production runs of different products.
The resilient contact between the printing plate and substrate to be printed is provided at least in part and/or significantly affected by the compressibility or resiliency of the tape. It is known in the art to use a double sided tape having a carrier containing a flexible foam layer and opposed layers of pressure sensitive-adhesive. Such double coated foam carrier tapes generally provide sufficient compressibility to assure printing of relatively fine or small indicia. However, the foam carrier tapes tend to be too compressible for printing of relatively large indicia (e.g. indicia having border dimensions of one-eighth inch or more) and cause excessive ink flow on the substrate. For printing such large indicia, the prior art teaches the use of a less compressible tape having a carrier containing a thermoplastic resin layer such as a polyvinylchloride film layer. Such vinyl carrier tapes tend to provide satisfactory printing of large indicia, but they do not provide the sharpness and fine detail needed in relatively small indicia. Consequently, printers have tended to inventory and use both foam and vinyl carrier tapes in order to attain satisfactory print quality for the full size range of print indicia.
In order to obtain uniform print quality, it is necessary that the tape be of uniform caliper or thickness. Variations in tape thickness tend to cause print color or intensity differences. Foam carrier tapes have tended to have unacceptable thickness variations, for example, a 20 mil thick tape may range from 16 to 24 mils in thickness. Such thickness variations yield undesirable print color or intensity differences as noted above. In extreme cases, thickness variations may prevent proper end-to-end abutting alignment of the printing plate around the print cylinder. For example, the two abutting tape edges may be at sufficiently different heights to cause a detectable printing difference across the butt joint.
The ease of tape removability from the print cylinder is important to efficient use of the tape and to maintaining good printing production rates. During installation of the printing plate, it may be necessary to remove and resposition the tape relative to the print cylinder. If the tape tears during its removal for repositioning, the tape is wasted and discarded without printing use. Following the printing process, the tape is removed from the print cylinder in preparation for the mounting of a different printing plate. In both cases, the tape should strip cleanly and separate from the print cylinder by hand manipulation without tearing or rupturing internal layers so as to leave behind irregular pieces of tape residue such as adhesive and/or tape fragments. Preferably, the tape pieces should strip from the cylinder as an integral piece without leaving behind tape residue and thereby provide xe2x80x9cone piece removabilityxe2x80x9d. Prior foam carrier tape constructions tended to shear or tear in the foam layer during removal from the cylinder and required additional time for tape removal and clean-up processing.
G.B. 1,533,431 discloses a laminated double sided tape for mounting flexographic letterpress printing plates including an elastomeric layer-having rigid-walled frangible bubbles, a dimensionally stable reinforcing layer and opposed outer adhesive layers. The frangible bubbles are broken with pressure and the wall fragments are retained in the resulting voids.
A double sided tape for mounting a flexible printing plate to a drum is disclosed in U.S. Pat. No. 4,574,697. The tape includes a polyurethane flexible foam applied to a polyethylene terephthalate base film and outer adhesive layers. The tape is indicated to be removable from the printing plate and drum as an integral sheet, and to be an improvement over vinyl carrier tapes that lack memory and do not retain gauge during long printing runs. A similarly constructed tape having a polyethylene foam layer selected to reduce vibrations is disclosed in European Patent Application 0 206 760.
U.S. Pat. No. 3,983,287 discloses a printing blanket comprising a laminate including a compressible interior support layer of incompressible elastomer having dispersed and bonded therein frangible rigid-walled bubbles that have been broken to impart compressibility.
As indicated above, the adhesive tape of the present invention may be used to detachably secure elements together. The tape includes a carrier and adhesive layers on opposite faces or sides thereof. The composition of the carrier layer is selected to achieve desired tape application characteristics as well as desirable manufacturing or processing benefits. The tape is especially useful in the mounting of flexographic printing plates.
The tape carriers are formed of polymeric materials that provide desired compressibility and tear resistance characteristics for flexographic printing applications while at the same time enabling economical tape manufacturing techniques. More particularly, the carrier comprises a multilayer coextrudate having an internal cellular structure. The carrier is formed by the coextrusion of a plurality of resin charges, at least one of the charges containing a blowing agent to provide the cellular structure.
In preferred constructions, the carrier comprises a closed cell core or central layer and at least one skin or outer layer. The core layer comprises a thermoplastic elastomer matrix containing a distribution of voids provided by the blowing agent. The skin layer comprises a continuous non-cellular, film-forming thermoplastic polymer layer that is preferably substantially free of internal voids as well as voids along the interface with the core.
The blowing agent preferably comprises a xe2x80x9cso-calledxe2x80x9d physical blowing agent in that it is thermally blown or expanded upon heating to an elevated temperature, but it does not chemically react to form the cellular structure. (A chemical blowing agent includes a chemical reaction to generate a gas that forms cells or voids in the polymer matrix.) The blowing agent expansion temperature may be matched with the extrusion temperature so that the blowing agent does not materially affect the extrusion processing of the core charge. In this manner, coextrusion processing may be used to simultaneously form the core in a single economical process operation without significant problems due to the presence of the blowing agent.
In the illustrated embodiment, the tape includes opposed layers of adhesive and at least one skin layer intermediate the carrier and one of the adhesive layers. The skin or skins separate the adhesive from the carrier layer, and restrict migration of adhesive constituents as well as the flow of printing ink solvents which may adversely affect the initial or aged tape properties.
The core provides the majority of the tape strength and compressibility properties. The skin smooths the adjacent outer core face which may be roughened excessively by the expansion of the blowing agent. The smoothing of the core face is by a leveling of the roughness resulting from the blowing process. In this manner, the skin provides the carrier with a smooth face.
Thermoplastic elastomers having a cellular structure in accordance with the invention are believed to enable a tape construction of printing compressibility characteristics suitable for the printing of both small and large indicia.
The cellular thermoplastic elastomer core layers of the invention also provide tapes of an internal tear strength or cohesive strength sufficient to enable tape repositionability and removability from a plate holder mounting surface such as a printing cylinder. The tape may be manually stripped from its installed position around the cylinder without layer tearing or tape residue adhering to the cylinder to enable repositioning of the tape or removal of the plate following completion of the printing process. The tape cleanly separates by manually pulling the tape from an end or edge thereof to provide. xe2x80x9cone piece removability.xe2x80x9d The tape also strips cleanly from the printing plate if it is necessary to reposition the plate during its installation.
The tape carrier outer layer is preferably coextruded in order to enhance the integrity of the multilayer carrier composite as noted above and to gain production economies over solvent coating techniques, for example. The cellular thermoplastic elastomer and the skin layers are therefore also selected and/or formulated to provide melt and eological properties enabling coextrusion. The tape including the adhesive layers may also be coextruded with selection and/or formulation of suitable layer properties.